A urea water is added to an upstream portion of an exhaust gas purifying device in an exhaust pipe to purify an exhaust gas emitted from an internal combustion engine. In a case where a concentration of the added urea water is lower than a proper concentration range, urea amount supplied to the exhaust gas becomes insufficient, which deteriorates an exhaust gas purification rate. In a case where the concentration of the added urea water is higher than the proper concentration range, an excessive amount of the urea is supplied to the exhaust gas, so that the urea which has not been reacted to the exhaust gas is discharged to atmosphere. Thus, the concentration of the urea water is necessary to be within the proper concentration range.
In a cold district, the urea water may solidify. Generally, there is a difference between a freezing point of water which is a solvent of the urea water and a freezing point of urea which is a solute of the urea water. Thus, the solvent and the solute start solidifying at different temperatures. Moreover, the freezing points of the solvent and the solute depend on the urea concentration. Since the specific gravity of solidified water is smaller than that of liquid-phase water, the solidified water flows up to a water surface of the water solution. Meanwhile, since the specific gravity of urea is larger than that of the water, the urea flows down to the bottom of the water solution. Thus, when the urea water starts solidifying, a concentration separation will occur.
When the engine is started with the urea water solidified, a part of the solidified urea water is thawed to be added into an exhaust pipe. In such a case, the thawed urea water also has the concentration separation. In order to avoid the concentration separation, 32.5% urea water has been used, as described in JP 2008-286096 A. 32.5% urea water has a eutectic point at −11° C. The eutectic point is a temperature at which urea and water solidify (crystalize) simultaneously. Therefore, it has been considered that 32.5% urea water has no concentration separation when solidifies.
However, from when the urea water starts solidifying until when all of the urea water in the storage tank totally solidifies to be in solid phase, the solid urea, the solid water and the water solution exist in the storage tank. According to the present inventors' study, it becomes apparent as follows. That is, due to a difference in specific gravity between the urea and the solidified water, the solidified water flows up to a water surface of the water solution, and the urea flows down to a bottom of the water solution. Thus, a concentration separation occurs in a perpendicular direction.
It is an object of the present disclosure to provide a urea water agitation control device which can restrict a solidification of the urea water with a concentration separation.
According to one aspect of the present disclosure, a urea water agitation control device has an agitation control portion which controls an agitation portion agitating the urea water stored in a storage tank, and a temperature sensor which detects a temperature of the urea water. The agitation control portion operates the agitation portion in a specified period which includes at least a part of a period in which the urea water temperature detected by the temperature sensor is kept at a eutectic point and the urea water radiates a latent heat of solidification.
The urea water is agitated in the specified period which includes at least a part of a period in which the urea water temperature detected by the urea water temperature sensor is kept at the eutectic point and the urea water radiates heat. Therefore, it can be restricted that solidified water and solidified urea are separately located at the water surface and the water bottom. It can be restricted that the urea water solidifies with a distribution of the concentration thereof (concentration separation).